The present invention is concerned with an audio codec supporting time-domain and frequency-domain coding modes.
Recently, the MPEG USAC codec has been finalized. USAC (Unified speech and audio coding) is a codec which codes audio signals using a mix of AAC (Advanced audio coding), TCX (Transform Coded Excitation) and ACELP (Algebraic Code-Excited Linear Prediction). In particular, MPEG USAC uses a frame length of 1024 samples and allows switching between AAC-like frames of 1024 or 8×128 samples, TCX 1024 frames or within one frame a combination of ACELP frames (256 samples), TCX 256 and TCX 512 frames.
Disadvantageously, the MPEG USAC codec is not suitable for applications necessitating low delay. Two-way communication applications, for example, necessitate such short delays. Owing to the USAC frame length of 1024 samples, USAC is not a candidate for these low delay applications.
In WO 2011147950, it has been proposed to render the USAC approach suitable for low-delay applications by restricting the coding modes of the USAC codec to TCX and ACELP modes, only. Further, it has been proposed to make the frame structure finer so as to obey the low-delay requirement imposed by low-delay applications.
However, there is still a need for providing an audio codec enabling low coding delay at an increased efficiency in terms of rate/distortion ratio. Advantageously, the codec should be able to efficiently handle audio signals of different types such as speech and music.
Thus, it is an objective of the present invention to provide an audio codec offering low-delay for low-delay applications, but at an increased coding efficiency in terms of, for example, rate/distortion ratio compared to USAC.